Hadfield steel strain hardening

Manganese hard steel only hardens through cold working on the surface. When welding, you have to be careful: the heat makes the heat-affected zone soft and tough, that's normal. The hardening only comes back later through mechanical stress in service. If you want to post-treat the weld, only work it cold – never with heat. And remember: the steel can't handle sharp notches, so grind the weld transitions nice and smooth.